YOUNG BIRDS AND RECORDS OF THE PAST 119 



finger-tip, on their under surface, recall the human thumb 

 and finger-tip in that the under surface forms a sort of 

 pad ; and these digits terminate each in a long claw. 

 With the fingers thus armed an additional grip is gained 

 of the boughs, a matter of very considerable importance 

 to the welfare of the climber. How important is the part 

 the hand plays may be gathered from the very curious 

 way in which the growth of the quill feathers belonging 

 to the finger-tip is suspended. 



Normally, it must be remembered, the quiU or flight 

 feathers in nidifugous, non-aquatic nestlings are pushed 

 on fairly rapidly, though not with the same express speed 

 which obtains among the game-birds. Now, if in the 

 young hoatzin all the quills of the hand started and con- 

 tinued to grow at the same rate, a stage would speedily 

 be reached when those at the tip of the finger would 

 seriously interfere with the gripping action of the finger- 

 tip and claw, while these feathers at the same time would 

 be useless for flight. Thus it happens that the growth of 

 these outermost quiUs is arrested until the inner feathers, 

 the quiUs nearer the wrist, are large enough to act as a 

 sort of parachute to break the force of a fall in the event 

 of such an accident. 



So soon as this stage of development is reached, cHmbing 

 is no more either necessary or possible, and a change 

 comes over the further development of the hand, a change 

 affecting the rate of growth of the whole wing, indeed. 

 In the first place the claws become slowly absorbed, while 

 the arm and fore-arm grow at a greater rate than the 

 hand, so that in consequence the hand speedily becomes 

 the shortest segment of the whole Jimb, while at first it 

 was by far the longest. This much will be gained by a 

 reference to the accompanying illustrations. By the 



